1. Field of the Invention
The present invention relates to alternative direct sequence spread spectrum symbol to chip mappings for wireless subsystems and methods for generating the same.
2. Background of the Related Art
Proliferation of wireless devices using standard transmission protocols on the market may be both advantageous and disadvantageous from the point of view of development of certain wireless subsystems. On one hand, wide availability of commercial off-the-shelf (COTS) wireless transceiver chips may allow lowering hardware costs, thus making wireless approach economically feasible. On the other hand, rapidly increasing numbers of users in the unlicensed frequency bands makes accidental interference a very likely problem.
In a typical case, a wireless communication system is operating in a frequency band divided into several channels. If transmission from another user is detected, it is then conceptually possible to switch to another, yet unoccupied channel. However, this channel switching approach may be not sufficiently reliable in practice. Synchronization of multiple devices is potentially difficult, particularly if the presence of interferers in the currently used channel makes it difficult to coordinate simultaneous switch of all concerned devices to the same channel.
An even more critical issue is the availability of unoccupied channels. Typically, the number of such channels in a given frequency band will be rather small. Therefore, it is quite possible that all of the channels will be occupied by other communication systems using the same protocol. As use of wireless communications becomes more widespread, the more likely it is that a channel will remain occupied, even for wireless subsystems that require immediate access.
The issue of overcrowding the available channels by multiple users is particularly significant for communication protocols that employ collision avoidance of simultaneously transmitted signals through clear channel assessment (CCA). For example, the 802.15.4 IEEE standard uses the CCA approach to detect if a transceiver may start sending data as described in the Institute of Electrical and Electronics Engineers, Inc., IEEE Std. 802.5.4-2003, IEEE Standard for Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Specific Requirements—Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low Rate Wireless Personal Area Networks (WPANs). New York: IEEE Press 2003.
Depending on the particular CCA mode employed at the moment, the transceiver first listens to the channel and tries to detect either energy above a threshold, or any signal using the 802.15.4 modulation protocol, or both. If such a source is detected, the transceiver backs off for a random time and tries again. This means that the presence even of an extremely weak signal source may prevent a transceiver from being able to initiate its own transmission for an unknown amount of time. In the worst-case scenario, the transceiver may wait forever for the other source to stop emitting its signal. This inherent delay is known as CCA vulnerability of a wireless system. In the case of wireless aircraft subsystems, CCA vulnerability is highly undesirable. Aircraft subsystems such as, flight control systems, aircraft navigation systems or even in-flight entertainment systems, for example, can potentially experience an unknown time restriction on the ability to transmit wirelessly. And, because of the stringent reliability requirements for these aircraft subsystems and other analogous systems, even occasional delay or cancellation of data transmission due to collision avoidance techniques, like CCA, may be completely unacceptable.
In a system with no economic constraints, using a non-COTS proprietary communication protocol created specifically for a given subsystem, either with no CCA mechanism at all or such that only detects transmissions using the proprietary protocol designed only for a given wireless system would yield significantly improved reliability. However, in view of the relatively low production volumes for some wireless subsystems, like wireless aircraft subsystems, for example, the necessary development investment makes this proprietary approach economically infeasible. In view of these concerns, a modification of an existing standard protocol that would allow use of most of the already existing infrastructure while simultaneously reducing the negative impact of CCA vulnerability is an alternative. One such modification is that of an alternative symbol to chip mapping that may be embedded in a transceiver used by a wireless subsystems that employ direct sequence spread spectrum signal modulation.